Comment on NanoVNA vs. Loop antenna SWR testing [Question]
einfach_orangensaft@sh.itjust.works 4 days agoThis is my capacitor (RG58):
I dont have ferrite beads right now sadly, but i will try to make a Air-Core choke from parts of the coax feed line, maybe that helps.
If i would use a balun between feed line and antenna, i need a 1:1 balun i assume?
I will try to find the frequency where the impedance gets real. Also gona try to maybe build a variable capacitor from 2 metal pipes going into each other depth regulated by a screw, but i mostly wana use this antenna on a single frequency so i hope i dont need much adjustment.
Just a remark, remember that everything that you connect to the radio and which is not matched on both sides will have an effect that the radio “sees”.
So if you get a different result with and without calibrating in the cable, and the cable is used for the radio connection as well, the vna result that doesn’t include cable calibration shows what the radio will see.
My repositioning the primary loop i managed to get the SWR down to 1.5 :D
I just hoped i aint cooking my insides when transmitting while sitting next to the loop antenna, a online calculator said at 4W i should keep at least 13cm distance from the antenna, but that was for a dipole not a loop…i need a longer feedline to get some distance…
I currently at berlin/brandenburg area, i kinda dont think i will be able to make long distance connection till i upgrade my radio to one that supports SSB tx with 12W (AE5900 eventually i hope). Long term i hope to manage to get a setup that allows me to join the europe wide JS8CALL network on 27.245Mhz. JS8CALL supports message forwarding over other nodes, a bit like meshtastic, i heared of people who managed to comunicate to australia via JS8CALL using multiple forwarding nodes, it is very slow tho due to using the same mode as FT8
My repositioning the primary loop i managed to get the SWR down to 1.5 :D
HF is black magic. Don’t let anyone convince you otherwise.
currently at berlin/brandenburg area
Ah, damn. I am in central / southern germany, a bit too far for antenna experiments :)
I just hoped i aint cooking my insides
To do some back-of-the-envelope math: Assuming a very much worst-case gain of 6 dB and 12 W actual output power that all makes it to the antenna and all gets radiated (it won’t):
12 W * 10^0.6 = 48 W
In free space, ignoring near-field weirdness (which you really shouldn’t, especially not with a mag loop!) that would be:
P = E^2 /Z ; E = (P*Z)^0.5 ; (48W * 377R)^0.5 = 135 V/m
The BImSchV (nice name, isn’t it) says a max peak field strength of 5 kV/m and a quadratic average over 6 minutes of 28 V/m is allowed. So, back of the envelope says, running for short periods of time next to your chair is “probably ok”.
Out of curiosity, I asked the official BNetzA WattWächter. It tells me to keep a safety distance of 4m for a mag-loop at 12W, but it only has mag-loops for 7 MHz and below. It has a CB version, but it doesn’t run for me, and I am too lazy to debug some stupid java app.
Note that with mag-loops, the near field is pretty “special” in that it has a pretty high magnetic field created by the large reactive currents between the capacitor and the loop inductance. As a rule of thumb, everything within 1-2 wavelengths of distance (so like 20 m for CB) is near-field weirdness. But don’t quote me on all that, I never looked into this too deeply.
Now, I say about myself that legal limits are boring and that there are several lifestyle decisions to take before I should worry about non-ionising fields, like no more alcohol, more sleep, more exercise… but I am an engineer, not a doctor, so I am not really qualified to talk about this kind of stuff. Make the health decisions you are comfortable with.
Ah yeah sad, southern germany is a bit far away (besides maybe during high solar activity but even they the skips may fly just right over). Thx for doing some calculations, on the safety distance aspect of it, i kinda assume the psychological effect (knowing that u in the RF near field) is probably bigger an the real effect of 4W of RF heating…and i probably just hallucinated the slight metallic taste in my mouth after 1min of keying down while tx testing (my brain do be like that sometimes).
fullsquare@awful.systems 4 days ago
I wouldn’t bother with air core. Ferrite beads allow you to use old calibration. If you make 1:1 balun just by threading coax through toroid, you can use old calibration as well provided it’s the same coax. Keep in mind minimum bending radius of coax. There are other designs, like using twisted pair on toroid
I’ve seen people using PE-Al-PE pipe for variables, this gives you layer of good dielectric (polyethylene) in dimensionally stable form. One connection is aluminum layer inside the pipe, and for the other you’ll have to figure it out on your own. Retuning might be required anyway within the band (magloops are narrowband) Common way to make variables is to bolt two of them in series, so that no sliding contact is used, moving part is the same for both. This is good for high voltages also but i’m not sure if you’ll need it